Field of the Invention
The present invention relates to a method of signal transmitting, by each cluster head, a synchronization signal at each transmission frame in a cluster-tree structured wireless communication system.
Description of Related Art
In a wireless sensor network (WSN), since each sensor device may use a limited power source, such as a battery, a communication technology reducing power consumption is necessary. For example, in IEEE 802.15.4 that is a type of WSN medium access control (MAC) method, power consumption may be reduced by enabling devices in a system to transmit and receive a signal only in an interval for transmitting data signal, referred to a DATA interval, and stopping the devices from operating in a following idle interval, referred to an IDLE interval, by using a beacon-enabled mode that uses a periodic transmission frame including an interval for transmitting a synchronization signal, referred to a SYNC interval, the DATA interval, and the IDLE interval. Here, a synchronization signal, referred to a beacon signal, is periodically transmitted for synchronization of the transmission frame between the devices in the WSN.
When a plurality of wireless communication systems of different types or same type, which share the same frequency band, exist, a direct sequence spread spectrum (DSSS) system reduces an effect of interference signal within a band with the aid of spread spectrum technology. For example, in IEEE 802.15.4 that is a type of the DSSS system, a transmission signal may be spread and transmitted so as to obtain a processing gain with respect to interference signals. However, the performance of IEEE 802.15.4 DSSS may not be effective when a frequency-static interference signal occurring for a relatively long time in a wide frequency band, such as a wireless local area network (WLAN), exists. In detail, in the beacon-enabled mode of IEEE 802.15.4, since the beacon signal is transmitted once at each transmission frame, when an interference signal exists in a channel being in use, a transmission performance of the beacon signal may remarkably deteriorate and thus network devices experience severe power consumption and performance degradation due to repeated attempts to receive a beacon signal. When a network device does not consecutively receive a beacon signal, it may lose synchronization with its access point (AP), determining itself as an orphan device. In the presence of interference signal, network connectivity may seriously deteriorate in the beacon-enabled mode of IEEE 802.15.4.
In order to prevent such problems of the beacon-enabled mode of IEEE 802.15.4, IEEE 802.15.4e MAC enhancement uses a deferred beacon method (DBM) that is a method for transmitting a synchronization signal with an aid of channel sensing. In the DBM, an AP performs channel sensing before transmitting its synchronization signal. The AP transmits its synchronization signal only when it confirms that a channel is not being used, alleviating a collision between its synchronization signal and an interference signal. However, since DBM depends only on the channel sensing of the AP, its performance may significantly deteriorate in a hidden node environment where only network devices are affected by interference signal. Also, since a beacon signal is transmitted only once at each transmission frame, the performance improvement may be marginal in severe interference environments.
In order to solve such problems of existing techniques, a method of repeatedly transmitting a synchronization signal has been suggested (KR 10-1481867). According to the method, an AP and network devices may determine the number of repeated transmissions of the synchronization signal in consideration of estimated channel characteristics and target transmission performance of the system, and the AP repeatedly transmits its synchronization signal, improving the synchronization performance of its network devices. However, according to the method, since the AP and the network devices perform a channel scan to estimate channel characteristics, power consumption by the network devices may increase. Also, according to the method, since the channel scan is always performed before the number of repeated transmissions of the synchronization signal is adjusted, power consumption by the network devices can be large. Lastly, according to the method, since a time interval for the channel scan is not accurately designed, a signal of the network devices existing in the same network may be mistaken as an interference signal, and thus the channel characteristics may incorrectly be estimated. Accordingly, when the method is applied to a cluster-tree structured multi-hop wireless communication network, the number of repeated transmissions of the synchronization signal may be excessively high and a channel hand-off may frequently be performed.